initiation and control of the heartbeat

• at the top of the right atrium, near the point where the vena cava empties the blood into the atrium, is the sino atrial node (SAN). This is a small patch of tissue that generates electrical activity. The SAN initiates a wave of excitation at regular intervals, in a human this occurs about 55-80 times per minute. the SAN is also known as the heart's pacemaker.
• describe what happens when the atria contract?

  A wave of excitation is quickly spread over the walls of both atria from the sinoatrial node. It travels along the membranes of the muscle tissue. as the wave of excitation passes, it causes the cardiac muscle cells to contract. This is known as atrial systole.
• The tissue at the base of the atria is unable to conduct the wave of excitation, and so it cannot spread directly down to the ventricle walls. at the top of the interventricular septum, is another node called the atrio ventricular node (AVN). This is the only route that can conduct the wave of excitation through to the ventricles. the wave of excitation is delayed in the AVN- this allows time for the atria to finish contracting and for the blood to flow down into the ventricles before they begin to contract.
• describe what happens when the ventricles contract?


  After a short delay, a wave of excitation is carried away from the atrioventricular node (AVN) and down the purkyne fibres. The fibres run up the apex of the heart, and down the septum. At the base of the septum, the wave of excitation spreads upwards from the base of the ventricles causing the muscle to contract. This pushes the blood up towards the major arteries at the top of the heart.